The following description is provided solely to assist the understanding of the present invention. None of the references cited or information provided is admitted to be prior art to the present invention. All patents and other references cited in the specification are incorporated by reference in their entireties, including any tables and figures, to the same extent as if each reference had been incorporated by reference in its entirety individually.
Cardiovascular disease is the leading cause of death in the United States. The most commonly used and accepted methods for determining risk of future heart disease include determining serum levels of cholesterol and lipoproteins, in addition to patient demographics and current health. The terms “lipoprotein” and “lipoprotein particle” as well known in the art refer to particles obtained from mammalian blood which include apolipoproteins biologically assembled with noncovalent bonds to package for example, without limitation, cholesterol and other lipids. Lipoproteins preferably refer to biological particles having a size range of 7 to 120 nm, and include VLDL (very low density lipoproteins), IDL (intermediate density lipoproteins), LDL (low density lipoproteins), Lp(a) [lipoprotein (a)], HDL (high density lipoproteins) and chylomicrons as defined herein. “Biological particle” refers to a material having a non-covalently bound assembly of molecules derived from a living source. Examples without limitation of biological particles are lipoproteins assembled for example from apolipoproteins and lipids; viral components assembled from non-covalently bound coat proteins and glycoproteins; immune complexes assembled from antibodies and their cognate antigens, and the like. Lipoprotein density can be determined directly by a variety of physical biochemical methods well known in the art, including without limitation equilibrium density ultracentrifugation and analytic ultracentrifugation. Lipoprotein density may also be determined indirectly based on particle size and a known relationship between particle size and density. Lipoprotein size may be determined by a variety of biochemical methods well known in the art including, without limitation, methods described herein. The term “apolipoprotein” refers to lipid-binding proteins which constitute lipoproteins. Apoliproteins are classified in five major classes: Apo A, Apo B, Apo C, Apo D, and Apo E, as known in the art. There are well established recommendations for cut-off values for biochemical markers, for example without limitation cholesterol and lipoprotein levels, for determining risk. The terms “marker,” “biochemical marker” and like terms refer to naturally occurring biomolecules (or derivatives thereof) with known correlations to a disease or condition. However, cholesterol and lipoprotein measurements are clearly not the whole story because as many as 50% of people who are at risk for premature heart disease are currently not encompassed by the ATP III guidelines (i.e., Adult Treatment Panel III guidelines issued by the National Cholesterol Education Program and the National Heart, Lung and Blood Institute).
Methods to measure lipoprotein and other lipids in the blood include, for example without limitation, evaluation of fasting total cholesterol, triglyceride, HDL (high density lipoprotein) and/or LDL (low density lipoprotein) cholesterol concentrations. Currently, the most widely used method for measuring LDL cholesterol is the indirect Friedewald method (Friedewald, et al., Clin. Chem., 1972, 18:499-502). The Friedewald assay method requires three steps: 1) determination of plasma triglyceride (TG) and total cholesterol (TC), 2) precipitation of VLDL (very low density lipoprotein) and LDL, and 3) quantitation of HDL cholesterol (HDLC). Using an estimate for VLDLC as one-fifth of plasma triglycerides, the LDL cholesterol concentration (LDLC) is calculated by the formula: LDLC=TC−(HDLC+VLDLC). While generally useful, the Friedewald method is of limited accuracy in certain cases. For example, errors can occur in any of the three steps, in part because this method requires that different procedures be used in each step. Furthermore, the Friedewald method is to a degree indirect, as it presumes that VLDLC concentration is one-fifth that of plasma triglycerides. Accordingly, when the VLDL of some patients deviates from this ratio, further inaccuracies occur.
Another method for evaluating blood lipoproteins contemplates measurement of lipoprotein size and density. The size distribution of lipoproteins varies among individuals due to both genetic and nongenetic influences. The diameters of lipoproteins typically range from about 7 nm to about 120 nm. The term “about” in the context of a numerical value represents the value +/−10% thereof. In this diameter size range, there exist subfractions of the particles that are important predictors of cardiovascular disease. For example, VLDL transports triglycerides in the blood stream; thus, high VLDL levels in the blood stream are indicative of hypertriglyceremia. These subfractions can be identified by analytical techniques that display the quantity of material as a function of lipoprotein size or density.
Regarding lipoprotein density analysis, ultracentrifugally isolated lipoproteins can be analyzed for flotation properties by analytic ultracentrifugation in different salt density backgrounds, allowing for the determination of hydrated LDL density, as shown in Lindgren, et al, Blood Lipids and Lipoproteins: Quantitation Composition and Metabolism, Ed. G. L. Nelson, Wiley, 1992, p. 181-274, which is incorporated herein by reference. For example, the LDL class can be further divided into seven subclasses (see Table 1) based on density or diameter by using a preparative separation technique known as equilibrium density gradient ultracentrifugation. It is known that elevated levels of specific LDL subclasses, LDL-IIIa, IIIb, IVa and IVb, correlates closely with increased risk for CHD (i.e., coronary heart disease), including atherosclerosis. Furthermore, determination of the total serum cholesterol level and the levels of cholesterol in the LDL and HDL fractions are routinely used as diagnostic tests for coronary heart disease risk. Lipoprotein class and subclass distribution is a more predictive test, however, since it is expensive and time-consuming, it is typically ordered by physicians only for a limited number of patients.
With respect to measurement of the sizes of lipoproteins, currently there is no single accepted method. Known methods for measuring the sizes of lipoproteins within a clinical setting include the vertical auto profile (VAP) (see e.g. Kulkarni, et al., J. Lip. Res., 1994, 35:159-168) whereby a flow analyzer is used for the enzymatic analysis of cholesterol in lipoprotein classes separated by a short spin single vertical ultracentrifugation, with subsequent spectrophotometry and analysis of the resulting data. Another method (see e.g. Jeyarajah, E. J. et al., Clin Lab Med., 2006, 26:847-70) employs nuclear magnetic resonance (NMR) for determining the concentrations of lipoprotein subclasses. In this method, the NMR chemical shift spectrum of a blood plasma or serum sample is obtained. The observed spectrum of the entire plasma sample is then matched by computer means with known weighted sums of previously obtained NMR spectra of lipoprotein subclasses. The weight factors that give the best fit between the sample spectrum and the calculated spectrum are then used to estimate the concentrations of constituent lipoprotein subclasses in the blood sample. Another method, electrophoretic gradient gel separation (see e.g. U.S. Pat. No. 5,925,229 incorporated by reference herein) is a gradient gel electrophoresis procedure for the separation of LDL subclasses. The LDL fractions are separated by gradient gel electrophoresis, producing results that are comparable to those obtained by ultracentrifugation. This method generates a fine resolution of LDL subclasses, and is used principally by research laboratories. However, the gel separation method, which depends on uniform staining of all components that are subsequently optically measured, suffers from nonuniform chromogenicity. That is, not all lipoproteins stain equally well. Accordingly, the differential stain uptake can produce erroneous quantitative results. Additionally, the nonuniform chromogenicity can result in erroneous qualitative results, in that measured peaks may be skewed to a sufficient degree as to cause confusion of one class or subclass of lipoprotein with another. Furthermore, gradient gel electrophoresis can take many hours to complete. It would be useful if gradient gel electrophoresis separation times could be shortened and the analysis simplified so that high resolution lipid analysis could be used in clinical laboratories as part of a routine screening of blood samples, and for example to assign a risk factor for cardiovascular disease.
Accordingly, a high-resolution methodology for measuring all subclasses of LDL as well as VLDL, IDL (intermediate density lipoprotein), HDL, Lp(a) and chylomicron particles that is accurate, direct, and complete, would be an important innovation in lipid, including lipoprotein, measurement technology. If inexpensive and convenient, such an assay could be employed not only in research laboratories, but also in a clinical laboratory setting. Ideally, clinicians could use this information to improve current estimation of coronary disease risk and make appropriate medical risk management decisions based on the assay.
Indeed, more recent methods for the quantitative and qualitative determination of lipoproteins from a biological sample have been described by Benner et al. (U.S. Pub. App. No. 2003/0136680, filed Nov. 12, 2002, and incorporated by reference in its entirety herein) which methods employ particulate size and/or ion mobility devices.
Ion mobility, also known as ion electrical mobility or charged-particle mobility, analysis offers an advantage over the other methods described herein in that it not only measures the particle size accurately based on physical principles but also directly counts the number of particles present at each size, thereby offering a direct measurement of lipoprotein size and concentration for each lipoprotein. Ion mobility analysis has been used routinely in analyzing particles in aerosols, and analyzers suitable for ion mobility analysis have been adapted to analyze large biological macromolecules. See e.g. Benner et al. (Id.) Ion mobility analysis is a very sensitive and accurate methodology with, nonetheless, a drawback that ion mobility analysis measures all particles introduced into the system. Accordingly, it is of prime importance to isolate and/or purify the compounds of interest prior to analysis. Lipoproteins are candidates for this method because lipoproteins can be isolated from other serum proteins based on density and other features described herein. Accordingly, by the present invention there are provided methods for purification and isolation of biomolecules including, without limitation, lipoproteins and biological complexes containing lipoproteins, for use in ion mobility analysis. The present invention further provides apparatus and methods for conducting ion mobility analyses.